A solution polymerization process for preparing a polyolefin using a metallocene catalyst, which corresponds to the technical field of the present invention, is typically a process of synthesizing a homogeneous polymer from a mixture comprising a solvent and at least one monomer selected from among alpha-olefins, such as ethylene, propylene, butene, hexene, octene and so on, in the presence of a metallocene catalyst. For reference, a metallocene catalyst is an organometallic compound configured to variously bind a transition metal (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, etc.) and a ligand derivative, having the basic structure of a cyclopentadiene, as disclosed in a variety of documents and patents, or any chemical structure that is an extension of the cyclopentadiene basic structure.
A typical metallocene catalyst-based solution polymerization process for preparing a polyolefin is a process in which a monomer, a solvent, a metallocene catalyst and so on are reacted with stirring in a polymerization reactor to afford a reaction product, which is then concentrated and solidified, resulting in a polymer.
During the solution polymerization process, the monomer is consumed, and the polymer, resulting from polymerizing the monomer, is dissolved in the solvent. As the concentration of the polymer increases, that is, as the polymerization progresses, the viscosity of the reaction mixture comprising the polymer, the unreacted monomer and the solvent is increased.
The reaction mixture is transferred from the polymerization reactor into a post-treatment device. The post-treatment device is used to separate volatile matter including the unreacted monomer and the solvent from the polymer of the reaction mixture. The removal of volatile matter is one of the final processing steps in the production of various polymers.
In the post-treatment device, the unreacted monomer and the solvent are gradually removed from the reaction mixture until the polymer obtained through the polymerization step is formed into a pellet or a bale. Here, the unreacted monomer and the solvent, which are separated, may be recirculated back to the polymerization reactor.
To remove volatile matter using the post-treatment device, a variety of methods of removing volatile matter from the reaction mixture depending on the viscosity of the reaction mixture are known, and may include heating a polymer solution to a temperature higher than the evaporation temperature of the volatile matter using a heat exchanger, or evaporating volatile matter from the high-temperature high-pressure reaction mixture in a vacuum or at low pressure.
Conventionally useful post-treatment processing is performed in a manner in which one or two flash drums are connected so that the solvent is sequentially removed from the reaction mixture. More specifically, the polymer solution resulting from the polymerization step is heated to a high temperature using a heat exchanger under predetermined pressure, placed in flash drums so that the solution is primarily volatilized at a low pressure to afford a dense polymer solution, from which the solvent is then secondarily volatilized at a lower pressure to yield a denser high-viscosity solution, which is then transferred into an extruder via a gear pump so that residual volatile matter is finally removed in a high vacuum, followed by a pelletizing step.
Alternatively, primary and secondary flash drum processes are omitted, and the solution may be treated using a single drum and then transferred into an extruder using a gear pump or gravity.
The case where the solution is transferred using gravity may be problematic because it is difficult to transport a high-molecular-weight polymer product, or a polymer having a specific molecular weight cannot be transported when the concentration thereof decreases below a predetermined level.
The present invention is undertaken to propose a technique for a novel polymer separation apparatus for producing a polymer product in which the amount of residual volatile matter is lower even without the use of a long extruder, which typically operates in a high vacuum, and for solving problems with the conventional art.
The related art to which the present invention belongs is briefly described below, and the technical features of the present invention different from those of the related art are also described.
Korean Patent No. 0769774 (Oct. 17, 2007) discloses a continuous solution polymerization method and apparatus, the apparatus comprising a pressure supply source, a polymerization reactor disposed downstream of the pressure supply source, a pressure-dropping device disposed downstream of the polymerization reactor, and a separator disposed downstream of the pressure-dropping device, wherein the pressure supply source functions to apply pressure to the reaction mixture during the operation of the processing apparatus, whereby a single-phase liquid reaction mixture is produced in the reactor and a two-phase liquid-liquid reaction mixture is produced in the separator, even without the use of an additional pressure supply source between the reactor and the separator.
European Patent No. 226204 (Mar. 3, 1993) discloses a process for removing volatile matter from a polymer solution containing 25 wt % or more of a polymer and a heat exchanger therefor, wherein the polymer solution is heated in an indirect heat-exchange zone comprising a plurality of channels. Here, the channels have a substantially consistent surface-area-to-volume ratio of 0.158 to 1.97 mm−1, a height of 1.27 to 12.7 mm, a width of 2.54 to 10.16 cm and a length of 1.27 to 30.48 cm, and the polymer solution is heated to a temperature that is higher than the evaporation temperature of volatile matter but is lower than the boiling temperature of the polymer at a pressure ranging from 2 to 200 bar within the channels, after which the solution from which 25% or more of volatile matter has been stripped is placed in a chamber. This process is advantageous because the time during which the polymer is exposed to the increased temperature is decreased, thus reducing thermal damage, but is disadvantageous because the solvent cannot be completely removed through a single step, and polymer deposits are formed outside a bundle of heat exchange tubes and may be carbonized over time, or may sometimes peel off in flakes, undesirably contaminating the solvent-free polymer.
Also, European Patent Application Publication No. 352727 (Jan. 5, 1994) discloses a method of removing volatile matter from a polymer solution by heating the polymer solution to a temperature higher than the evaporation temperature of volatile matter via a plurality of channels that are connected in parallel. More specifically, the ratio of the heat exchange surface area to the volumetric flow rate of the product is about 80 m2/m3/h, the flow rate in the channels is about 0.5 mm/s, and the residence time of the polymer solution in the channels ranges from 120 to 200 sec. This process is problematic because the solvent cannot be completely removed through a single step, and polymer deposits are formed outside a bundle of heat exchange tubes and may be carbonized over time, or may sometimes peel off in flakes, undesirably contaminating the solvent-free polymer.
Such related documents are somewhat similar to the present invention in that a solution polymerization process is employed to prepare a polyolefin using a metallocene catalyst, but are distinguished from the present invention because the effective separation of a polymer using the combination of a flash drum and a thin-film evaporator, corresponding to the gist of the present invention, is not found in the related documents.